diff --git a/framework/db/dao/Driver.php b/framework/db/dao/Driver.php
index 387505a..e183749 100644
--- a/framework/db/dao/Driver.php
+++ b/framework/db/dao/Driver.php
@@ -29,6 +29,7 @@ abstract class Driver extends \yii\base\Object
 	/**
 	 * The followings are the supported abstract column data types.
 	 */
+	const TYPE_PK = 'pk';
 	const TYPE_STRING = 'string';
 	const TYPE_TEXT = 'text';
 	const TYPE_SMALLINT = 'smallint';
diff --git a/framework/db/dao/mysql/ColumnSchema.php b/framework/db/dao/mysql/ColumnSchema.php
deleted file mode 100644
index f292d3e..0000000
--- a/framework/db/dao/mysql/ColumnSchema.php
+++ /dev/null
@@ -1,107 +0,0 @@
-<?php
-/**
- * ColumnSchema class file.
- *
- * @author Qiang Xue <qiang.xue@gmail.com>
- * @link http://www.yiiframework.com/
- * @copyright Copyright &copy; 2008-2012 Yii Software LLC
- * @license http://www.yiiframework.com/license/
- */
-
-namespace yii\db\dao\mysql;
-
-/**
- * ColumnSchema class describes the meta data of a MySQL table column.
- *
- * @author Qiang Xue <qiang.xue@gmail.com>
- * @since 2.0
- */
-class ColumnSchema extends \yii\db\dao\ColumnSchema
-{
-	/**
-	 * Extracts the PHP type from DB type.
-	 * @param string $dbType DB type
-	 */
-	public function initTypes($dbType)
-	{
-		static $typeMap = array(  // dbType => type
-			'tinyint' => self::TYPE_SMALLINT,
-			'bit' => self::TYPE_SMALLINT,
-			'smallint' => self::TYPE_SMALLINT,
-			'mediumint' => self::TYPE_INTEGER,
-			'int' => self::TYPE_INTEGER,
-			'integer' => self::TYPE_INTEGER,
-			'bigint' => self::TYPE_BIGINT,
-			'float' => self::TYPE_FLOAT,
-			'double' => self::TYPE_FLOAT,
-			'real' => self::TYPE_FLOAT,
-			'decimal' => self::TYPE_DECIMAL,
-			'numeric' => self::TYPE_DECIMAL,
-            'tinytext' => self::TYPE_TEXT,
-            'mediumtext' => self::TYPE_TEXT,
-            'longtext' => self::TYPE_TEXT,
-            'text' => self::TYPE_TEXT,
-            'varchar' => self::TYPE_STRING,
-            'string' => self::TYPE_STRING,
-            'char' => self::TYPE_STRING,
-			'datetime' => self::TYPE_DATETIME,
-			'year' => self::TYPE_DATE,
-			'date' => self::TYPE_DATE,
-			'time' => self::TYPE_TIME,
-			'timestamp' => self::TYPE_TIMESTAMP,
-			'enum' => self::TYPE_STRING,
-		);
-
-		$this->dbType = $dbType;
-		$this->type = self::TYPE_STRING;
-		$this->unsigned = strpos($this->dbType, 'unsigned') !== false;
-
-		if (preg_match('/^(\w+)(?:\(([^\)]+)\))?/', $this->dbType, $matches)) {
-			$type = $matches[1];
-			if (isset($typeMap[$type])) {
-				$this->type = $typeMap[$type];
-			}
-
-			if (!empty($matches[2])) {
-				if ($type === 'enum') {
-					$values = explode(',', $matches[2]);
-					foreach ($values as $i => $value) {
-						$values[$i] = trim($value, "'");
-					}
-					$this->enumValues = $values;
-				} else {
-					$values = explode(',', $matches[2]);
-					$this->size = $this->precision = (int)$values[0];
-					if (isset($values[1])) {
-						$this->scale = (int)$values[1];
-					}
-					if ($this->size === 1 && ($type === 'tinyint' || $type === 'bit')) {
-						$this->type = 'boolean';
-					} elseif ($type === 'bit') {
-						if ($this->size > 32) {
-							$this->type = 'bigint';
-						} elseif ($this->size === 32) {
-							$this->type = 'integer';
-						}
-					}
-				}
-			}
-		}
-
-		$this->phpType = $this->extractPhpType();
-	}
-
-	/**
-	 * Extracts the default value for the column.
-	 * The value is typecast to correct PHP type.
-	 * @param mixed $defaultValue the default value obtained from metadata
-	 */
-	public function initDefaultValue($defaultValue)
-	{
-		if ($this->type === 'timestamp' && $defaultValue === 'CURRENT_TIMESTAMP') {
-			$this->defaultValue = null;
-		} else {
-			$this->defaultValue = $this->typecast($defaultValue);
-		}
-	}
-}
\ No newline at end of file
diff --git a/framework/db/dao/mysql/Driver.php b/framework/db/dao/mysql/Driver.php
index 6dbb0d4..b995e88 100644
--- a/framework/db/dao/mysql/Driver.php
+++ b/framework/db/dao/mysql/Driver.php
@@ -22,7 +22,7 @@ use yii\db\dao\ColumnSchema;
 class Driver extends \yii\db\dao\Driver
 {
 	/**
-	 * @var array mapping from physical types (keys) to abstract types (values)
+	 * @var array mapping from physical column types (keys) to abstract column types (values)
 	 */
 	public $typeMap = array( // dbType => type
 		'tinyint' => self::TYPE_SMALLINT,
@@ -260,4 +260,14 @@ class Driver extends \yii\db\dao\Driver
 		}
 		return $names;
 	}
+
+	/**
+	 * Creates a query builder for the database.
+	 * This method may be overridden by child classes to create a DBMS-specific query builder.
+	 * @return QueryBuilder query builder instance
+	 */
+	public function createQueryBuilder()
+	{
+		return new QueryBuilder($this->connection);
+	}
 }
diff --git a/framework/db/dao/mysql/QueryBuilder.php b/framework/db/dao/mysql/QueryBuilder.php
index 35838f3..5f5934c 100644
--- a/framework/db/dao/mysql/QueryBuilder.php
+++ b/framework/db/dao/mysql/QueryBuilder.php
@@ -19,10 +19,10 @@ namespace yii\db\dao\mysql;
 class QueryBuilder extends \yii\db\dao\QueryBuilder
 {
 	/**
-	 * @var array the abstract column types mapped to physical column types.
+	 * @var array mapping from abstract column types (keys) to physical column types (values).
 	 */
     public $typeMap = array(
-		'pk' => 'int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY',
+		Driver::TYPE_PK => 'int(11) NOT NULL AUTO_INCREMENT PRIMARY KEY',
 		Driver::TYPE_STRING => 'varchar(255)',
 		Driver::TYPE_TEXT => 'text',
 		Driver::TYPE_SMALLINT => 'smallint(6)',